The optimum UV irradiation duration for the complete inactivation of Heteropneustes fossilis egg's nucleus was developed. The unfertilized eggs were suspended in Hanks' balanced salt solution and were irradiated with UV light from both the dorsal and the ventral side for different exposure times ranging from 20 to 200 s (2,500 to 25,000 ergs/mm2). Egg viability was assessed for the different exposure durations at fertilization, hatching, and haploidy. Majority of the larvae derived from irradiated eggs had an abnormal appearance. Complete inactivation of maternal genome was evaluated by haploid chromosome number and morphology which also exhibited haploid syndrome. Successful genome inactivation occurred from 100 s onwards. Larvae resulting from eggs irradiated above 12,500 ergs/mm2 were 100% haploid, with better hatching (81. 59%). The genetic inactivation of maternal chromosomes was confirmed by the chromosome count of the resulting embryo (n = 29). Completely anuclear embryos were obtained when the eggs were exposed with UV irradiation between 12,500 and 25,000 ergs/mm2. We conclude that the optimum UV dosage for the complete genetic inactivation of the egg nucleus requires more than 12,500 ergs/mm2. © 2012 Christopher et al; licensee Springer.